A common form of crane has a base, an upperstructure rotationally mounted on the base, a boom having a proximal end pivotally attached to the upperstructure and having a distal end supporting a load line for raising a payload, and a gantry mounted on the upperstructure and having an upper end supporting a boom line coupled to the distal end of the boom for supporting the distal end of the boom. This common form of crane is often provided with a motor to swing the boom around the base, a boom hoist to raise or lower the inclination of the boom to align the distal end of the boom over the payload, and a load hoist to reel-in or reel-out the load line in order to raise or lower the payload.
The common form of crane introduced above is often used for pedestal mounted offshore cranes. An example is designated as a “Swing bearing mounted lattice boom wire luffed crane” in FIG. 1 on page 2 of American Petroleum Institute Specification 2C, Offshore Pedestal Mounted Cranes, Seventh Edition, March 2012. In this example, the support base is a cylindrical pedestal, the upperstructure is generally rectangular, and the upperstructure is mounted to the pedestal via a swing-circle assembly for rotation about a vertical axis of the pedestal. The common gantry has a pair of rear legs mounted to the rear left and right of the upperstructure, and a pair of front legs mounted to the front left and right of the upperstructure, though different numbers of legs and different mounting positions for them do exist. The boom is a lattice boom pivotally connected to the front of the upperstructure. As a result, pedestal mounted offshore cranes typically use a four-legged gantry and six attachment locations on the upperstructure, including two attachment locations for a rear pair of gantry legs, two attachment locations for a front pair of gantry legs, and two attachment locations for a pair of boom legs at the proximal end of the boom. See, for example, Bonneson et al. U.S. Pat. No. 4,216,870 issued Aug. 12, 1980.